php - Registration form returning Server Error 500

I've created a form where users input their registration details and it enters it into an SQL database using a query. However, I type in all the details, click submit and it return a server error 500.
I'm new to binding & executing so probably slipped up with the format of the mysqli lines.

registrationsubmit.php

include 'registrationform.php';
include 'connection.php';
?>

if (isset($_POST['regsubmit'])) {
    $firstname = $_POST['firstname'];
    $lastname = $_POST['lastname'];
    $user = $_POST['username'];
    $pass = $_POST['password'];
    if ($stmt = mysqli_prepare($dbhandle, $query = "INSERT INTO users (FirstName, LastName, Username, Password) VALUES (?,?,?,?)")) {
    mysqli_stmt_bind_param($stmt, 'ssss', $firstname,$lastname,$user,$pass);
    mysqli_stmt_execute($stmt) or die('This username is already in use - Please choose another one');
 }else{
die('Error when preparing '.mysqli_error($dbhandle));
}
  Echo "Registration Successful - Press the Home button to return to the homepage.";
}
?>